Modulated carrier wave transmitter



April 15, 194

MODULATED CARRIER WAVE TRANSMITTER W. T. DITCHAM Filed Nov; 30, 1938MODULAT/NG POTEN TIA LS SOURCE OF OSC/LLA TIONS M ODJJLA TOP. AMPLIFIERAMPLIFIER 2 H 15 {9 23 .0AD

I 2 a I 25a i 25 AI MODULA TING POTENTIALS saunas 0F OSCILLA T/ONSMonyL/irolz AMPLIFIER 18 2/ l 15 /9 2e :ELLOAD 3] II 26b INVENTORAMpL/F/ER 26a BYW/LL/AM T 5 RE DITCHAM W/ ATTORNEY Patented Apr. 15,1941 MOBULATED CARRIER WAVE TRANSMITTER William Theodore Ditoham,Chelmsford, England, assignor to Radio Corporation of America, a

corporation of Delaware Application November 30, 1938, Serial No.243,111 In Great Britain December 1, 1937 6 Claims.

This invention relates to radio and other modulated carrier wavetransmitters and has for its object to provide improved modulatedcarrier wave transmitters wherein high efiiciency of the high poweramplifier-tubes employed is obtained.

According to this invention a load circuit which is to be energised withmodulated carrier wave energy is fed from the output circuits of twothermionic tube amplifiers one of which receives unmodulated carrierinput and has anode potential applied thereto through a modulationfrequency impedance of high value, the other of said amplifiersreceiving side band energy of the carrier wave on its grid together witha fixed bias and a carrier frequency input of such amplitude and phasethat zero or approximately zero anode current is obtained when theoscillations in the load circuit correspond to the unmodulatedcondition,said two amplifiers feeding into the common load circuit and being soarranged thatthe amplitude of the carrier frequency component/on theoutput electrode of said one amplifier is about twice that on the outputelectrode of said other amplifier.

In a modification theamplitude of the radio frequency on the anodes ofboth amplifiers is the same but the direct-current voltage for oneamplifier is twice the direct-current voltage on the anode of the otheramplifier so that the division of power between the amplifiers in thismodification and the prior modification is substantially the same.

The invention is illustrated in the accompanying drawing which showsdiagrammaticlly two embodiments thereof.

Referring first to Figure 1, output from a source |-a masteroscillator-4f carrier frequency oscillations is applied via a tunedtransformer 2 and coupling condenser 3 between the grid 4 and cathode 5of a triode or other high power thermionic tube 6 (hereinafter termedtheifirst main tube) which is so adjusted as regards the negative biasupon its grid and the anode voltage applied thereto that it operates asan eflicient so-called class C amplifier. A grid resistance 1 isconnected between grid 4 and cathode 5. The anode 8 of tube 6 receivesanode potential from any suitable D. C. source (not shown) through ahigh impedance audio frequency inductance 9 in series with a carrierfrequency choke It). The said anode 8 is coupled by means of a condenserH to one end of a carrier frequency parallel tuned oscillatory circuitI2, l3 whose other end is connected to the cathodefi and fearthed. Theinductance element 3 in this tuned circuit is coupled in any convenientway as by a coil M to a load circuit, such as a transmitting aerial, butwhich is represented conventionally by a resistance l5.

A second thermionic tube It (hereinafter termed the second main tube),for example a triode, has its anode connected to the positive terminalof any convenient D. C. source e. as indicated the same source as thatwhich supplies the first main amplifier tube 6, through a carrierfrequency choke l1, and its output circuit is coupled to the loadcircuit in such manher that the voltage component of carrier frequencybetween anode I8 and cathode IQ of the second main tube I6 is about halfthat which exists between anode 8 and cathode 5 of the first main tube6. For example, as shown the anode l8 may be capacity coupled by acondenser 20 to a point near the middle of the inductance l3 the cathodel9 being earthed and connected to the cathode 5.

The grid 2| of the second main amplifier I6 is negatively biassed to thepoint of anode current cut-oil for theparticular anode D. C. potentialemployed, this bias being applied from a source 22 in series with acarrier frequency choke 23. In addition there is applied between thegrid 2| and cathode |9 a carrier frequency potential of such amplitudeand phase that the anode current is zero or approximately zero foroscillations in the oscillatory circuit I2, I3 corresponding to thecarrier or unmodulated condition. Oscillations of side band frequencyare also applied between the grid 2| and cathode I!) of the second mainamplifier I6. For example, as shown, carrier wave oscillations may betaken from the carrier wave source I through a transformer 24 with amid-tapped secondary and applied to excite in phase opposition twothermionic tube amplifiers 25, 2B feeding into a carrier wave tunedcircuit 2'! included (in series with a coupling condenser 28) in thegrid cathode circuit of the second main amplifier it, the two outputsfrom the amplifiers 25 and 26 being combined in this tuned circuit 2'!in opposition. Any means known per se are provided for modulating theamplitude of the oscillations in one of the amplifiers 25, 26 so that,in the condition of balance as respects the carrier frequency in theseamplifiers, potentials of side band frequency only will reach the grid2| of the second main amplifier l6. As shown modulating potentials areapplied at 29 to a modulator 3|) associated with the amplifier .25. Inthe circuit of Figure 1 the output circuits 25a, 26a of the twoamplifiers 25, 26 may be coupled each to one or another of two seriescoils 25b, 261) which are together shunted by a condenser 21a toconsti-- tute the carrier frequency tuned circuit 21, this tuned circuitbeing coupled at one end by the condenser 28 to the grid 2| and beingconnected to the common cathode point of the two main amplifiers(hitherto assumed, as is usual, to be earth) at the other end. Thecoupling of the two amplifiers 25, 26 to the common tuned circuit 21 isslightly unbalanced to produce the requisite amplitude and. phase ofcarrier frequency required on the grid 2| of the second main amplifierI6.

Figure 2 shows a slight variation, which has been found in experimentalpractice to give somewhat better results, of the circuit of Figure l.The essential difierence between the arrangements of Figure 2 and Figurel is that, in Figure 2 the anodes 8, I8 of the two main amplifiers arecoupled to the same point on the tuned circuit I2, I3as shown the liveend thereof-and the anode I8 of the second main amplifier I6 is suppliedfrom a source of potential (indicated by two positive signs) of abouttwice the value of that from which the anode 8 of the amplifier 6 issupplied.

In the unmodulated condition the tube 6 supplies carrier power to theload I5 at high efiiciency, because it is biassed for class C operationand because the impedance of the circuits I2, I3, I4, I5 is such thatthe peak potential across I2, I3 is nearly equal to the supply D, C.potential. Note that the anode feed to 6 is supplied through theconstant current choke coil 9.

The tube I6 is biassed by 22 to such a value that the D. C. potential onits anode produces only a small anode current. Tube I6 is also driven bya carrier frequency potential of such amplitude and of such phaserelative to the carquency potentials applied to grid of tube I6 is isuch as to cause I6 to supply potentials to I2, I3 in phase with thosesupplied by tube 6 the potential across I2, I3 will increase. Thisincrease in potential would tend to reduce the feed current in 6, butowing to the presence of choke coil 9 this reduction is not possible atthe normal audio frequencies (30 to 10,000 cycles per second) andconsequently the supply potential on 6 rises eventually reaching twicethe static value. When the tube I6 supplies power to tank circuit I2, I3of the same phase as the power supplied from tube 6 to the tank circuitl2, I3 then to maintain a given voltage across the tank circuit lesscurrent need be supplied from tube 6. In other words, from the point ofview of tube 6, the efiect of tube I6 is exactly equivalent to anincrease in the impedance in the tank circuit I2, I3. In the presence ofa constant direct-current potential, such an increase in tank impedancewould be accompanied by a decrease in direct current but lation, that iswhen the phase of the side-band frequency potentials applied to grid oftube I6 is such as to cause I6 to supply potentials to I2, I3 in phaseopposition to those supplied by tube 6, or in another aspect to causetube I6 to absorb power from I2, I3, the potential across I2, I3 will bereduced. This reduction in potential would tend to increase the feedcurrent in 6 but owing to the presence of choke coil 9 this increase isnot possible and consequently the supply potential on 6 falls.

It should be obvious that if tube I6 is to be capable of supplying powerto circuit I2, I3 it must be connected at a point on this circuit ofwhich the potential is considerably less than the D. C. supplypotential, as is indicated in Figure 1, or alternatively must besupplied from a D. C. source of considerably higher voltage than the D.C. source for tube 6, as indicated in Figure 2.

The desirability of applying carrier frequency bias to grid of I6 is toimprove efficiency and linearity. If I6 was biassed to cut off merelyfor the D. C. supply potential, that is for linear class Bamplification, then when subjected to the carrier frequency potentialfrom I2, I3 a considerable current would flow causing loss of power. Ifon the other hand the static bias on grid of I6 was made high enough toprevent appreciable anode currentfiow under these conditions, then I6would not give linear amplification of the sideband energy,

What is claimed is:

1. A modulating system comprising, in combination a source of voltagesof carrier wave frequency, a class C amplifier fed from said source,means for preventing variation at modulation frequency of direct currentfed to said amplifier, a load circuit for said amplifier, a source ofmodulating potentials, means for deriving from said sources side bandenergy corresponding to the modulating potentials, a second amplifierhaving its input excited by said side band energy and its output coupledto the load circuit of said first amplifier, said latter coupling beingso adjusted that at maximum delivery of power to said load the ratio ofalternating plate voltage to direct current plate voltage issubstantially the same in both of said amplifiers.

2. In a modulation system, a tuned reactance, a source of wave energy ofa frequency to which said tuned reactance is of high impedance, a sourceof modulating potentials, an electron discharge tube having inputelectrodes connected to said source of wave energy and having an outputelectrode connected to said tuned reactance, means for biasing said tubefor class C operation, means for supplying substantially constant directcurrent to said output electrode, an electron discharge device havinginput electrodes and having output electrodes coupled to a portion ofsaid tuned reactance, means connected with said source of wave energyand with said source of modulating potentials for producing a carrierfrequency voltage and side band voltages, means for impressing saidcarrier and side band voltages on said input electrodes of said device,the phase of the carrier voltage so impressed being opposite to thephase of the voltage of the output electrode of said tube whereby saidside bands produce alternating currents in said reactance which opposethe alternating currents impressed on said reactance by said tube onmodulation swings in one direction and aid said alternating currents onthe modulation swings in the other direction.

3. In a modulation system, a load impedance, a source of wave energy tobe modulated in accordance with signals, a first tube havinginputelectrodes coupled to said source of wave energy and having outputelectrodes coupled to said load impedance and connected to a source ofsubstantially constant direct current, a discharge device having outputelectrodes connected to said load impedance and having input electrodes,a source of signal potential, modulating means connected with saidsource of signal potentials and said source of Wave energy for producinga carrier and side band energy, means for impressing said carrier andside band energy on the input electrodes of said device, the phase ofsaid carrier impressed on the input electrodes of said device beingsubstantially the same as the phase of the carrier supplied to saidtube, and means for biasing said device by a potential such that in thepresence of carrier energy only on said input electrodes of said devicethe anode alternating current supplied to said load impedance by saiddevice is substantially zero.

i. In a modulation system, a load impedance, a source of Wave energy tobe modulated in accordance with signals, a first tube having inputelectrodes coupled to said source of wave energy 5 and having an outputelectrode coupled to said load impedance and connected to a source ofsubstantially constant direct current, a discharge device having outputelectrodes connected to said load impedance and having input electrodes,a

source of signal potentials and means connecting said source of signalpotentials to said source of wave energy for producing a carrier andside band energy in a desired ratio, means for impressing said carrierand side band energy on the input electrodes of said device the phase ofsaid carrier impressed on the input electrodes of said device beingsubstantially opposed to the phase of the alternating voltage on theoutput electrode of the tube, and means for biasing said device by apotential such that in the absence of side band energy on the inputelectrodes of said device the anode alternating current supplied to saidimpedance by said device is substantially zero.

'5. In a modulation system, a source of wave energy to be modulated, asource of signalling potentials for modulating said wave energy, a firsttube having a control grid and cathode coupled to said source of Waveenergy and having an anode, a source of substantially constant directcurrent connected to the anode of said tube, a discharge device having acontrol grid, a cathode, and an anode, a circuit connecting the anodeand cathode of said device to a source of direct current potential, amodulator connected to said source of carrier wave energy and to saidsource of modulating potentials, means for deriving side band energy andcarrier energy from said modulator and impressing the same on saidcontrol grid and cathode of said discharge device, means for biasing thecontrol grid of said discharge device to a potential such that in theabsence of side band excitation negligible alternating current flows insaid device, and a load reactance coupled to the anode and cathode ofsaid tube and to the anode and cathode of said device, the voltageratios of said couplings being different. 6. A modulating systemcomprising, in comination a source of voltage of carrier Wave frequency,a class C amplifier fed from said source, means for supplying directcurrent to said amplifier, means for substantially preventing variationat modulation frequencyof direct current supplied to said amplifier, aload circuit for said amplifier, a source of modulating potentials,separate means for deriving from said source of voltage and source ofmodulating potentials side ban-d energy corresponding to the saidmodulating potentials, a second amplifier having its input excited bysaid side band energy and its output coupled to the load circuit of saidfirst amplifier, means for supplying direct current to said secondamplifier, said latter coupling being so adjusted that at maximumdelivery of power to said load the ratio of alternating plate voltage todirect current plate voltage is substantially the same in both of saidamplifiers.

WILLIAM THEODORE DITCHAM.

